A substantial number of graphic recording systems have been developed to provide automated or computer controlled graphic recording upon a media. While the individual structures of such graphic recorders vary substantially, generally all include a media transport system in which the media is moved in a first direction together with a moveable pen carriage supported above the media. Control means provide motion of the pen carriage in a second direction relative to the media. In most instances, the directions of media movement and pen carriage movement are orthogonal.
With the development of computer drive systems for such graphic recorders having increased capability and computing power, various systems have been provided which employ a plurality of recording pens supported upon the pen carriage. Generally, multiple pen systems are used to permit the selection from among a plurality of recording pens on the carriage having different colors. As an integral part of the operation of a multiple pen recording system, means are provided which selectively engage a single pen and move it to a position for recording upon the media. The remaining pens are maintained in positions out of contact with the media to ensure that only the selected pen engages and records upon the media.
With the introduction of multiple pen recording systems and the concurrent introduction of mechanisms to move the selected pen into contact with the media, there arose a need to accommodate various tolerances and variables within the recording system to ensure the accurate positioning of the pen or other recording element upon the media. Accurate pen to media contact is subject to several sources of difficulty which include different thicknesses of recording media and mechanical tolerances within the pen movement and control system. In addition, the majority of recording elements change point length due to wear during use. The combined effect of these variables requires that the position of the pen, with respect to the media, be periodically adjusted for optimum recording.
In addition to placement of the pen or recording element upon the media, the system requires that the appropriate pressure between the recording element and the media be applied and maintained to ensure proper recording. The latter requirement creates an additional variable within the system in that different combinations of media and recording element type require different contact pressures to assure proper recording. For example, recording pens having ball-type writing mechanisms generally require greater pen pressure against the media than those having porous fiber elements. These requirements have prompted practitioners in the art to devise various systems of pen motion and control in an attempt to accurately and reliably place the recording element against the media and apply the appropriate writing pressure. Such systems have included linear magnetic motor or voice coil moving mechanisms, or electrically operated solenoid coil systems to move the pen or recording element from its idle position above the media surface to contact with the media. In attempts to provide compensation for the above-described variables of system tolerances, recording element wear, and media thickness, practitioners in the art have often employed a spring biasing system within the pen movement mean to provide system " give" and maintain a predetermined spring force between the recording element and the media which in turn becomes the writing force of the recording system.
The above-referenced copending application sets forth an automatic pen positioning system in which a digital logic control system having the ability to compensate for the variables to which the system is subjected, such as system tolerances and media thickness, and compensate for wear of the recording element while applying a controlled precise contact pressure between the recording element and the media.
The system set forth in the above-referenced copending application is operative in a graphic recorder having and including a motor drive moving a recording pen into contact with recording media and a servo control system operative upon the motor drive to control pen motion. Limit means establish a recording pen travel reference point and move the recording pen toward the media at a predetermined velocity. Sensing means are coupled to the servo control system and detect the response of the servo control system to the obstruction of recording pen motion at media contact. A reference point is established when a recording pen obstruction is sensed which is used to control subsequent pen motion to media contact.
While the foregoing described systems provide some improvement in graphic recorder operation, their arises a further need for recording pen control systems which adjust for the writing characteristics of different recording pen types.